New types of lenses in optics: Researchers develop hybrid achromatic lenses with high focusing efficiency

Researchers at the University of Illinois at Urbana Champaign have developed compact visible wavelength achromatic mirrors using 3D printing and porous silicon, which are crucial for miniaturization and lightweight optical devices.

These high-performance hybrid micro optical devices can achieve high focusing efficiency while minimizing volume and thickness. In addition, these microlenses can be constructed into arrays to form larger images for use in achromatic light field imagers and displays.

This study was led by materials science and engineering professors Paul Braun and David Cahill, electrical and computer engineering professor Lynford Goddard, and former graduate student Corey Richards. The results of this study are published in Nature Communications.

Braun said, "We have developed a method that can create structures with classical compound optical functions but highly miniaturized and thin through non-traditional manufacturing methods.".

In many imaging applications, there are multiple wavelengths of light, such as white light. If a single lens is used to focus this type of light, different wavelengths are focused at different points, resulting in blurred image colors. To solve this problem, multiple lenses are stacked together to form an achromatic lens. "In white light imaging, if you use a single lens, you will have significant dispersion, so each component color is focused at different positions. However, when using an achromatic lens, all colors are focused at the same point," Braun said.

However, the challenge lies in the relatively thick stacking of lens components required for manufacturing achromatic lenses, which makes classical achromatic lenses unsuitable for newer and smaller technology platforms, such as ultra compact visible light wavelength phase machines, portable microscopes, and even wearable devices.
In order to form a thinner lens, the team combined a refractive lens with a planar diffractive lens. Braun explained that the bottom lens is a diffractive lens, for example, focusing red light closer, while the top lens is a refracting lens that can further focus red. They cancel each other out and focus on the same position.

In order to create a compact hybrid achromatic imaging system, researchers have developed a manufacturing process called subsurface controllable refractive index through beam exposure, where the polymer structure is 3D printed in a porous silicon main medium that mechanically supports optical components. In this process, liquid polymers are filled into porous silicon and converted into solid polymers using ultrafast lasers. Through this method, they are able to integrate diffraction and refractive elements of lenses without the need for external support, while minimizing volume, improving manufacturing convenience, and providing efficient achromatic focusing.

Richards explained, "If you print a lens in the air and want to stack two lenses together, you need to print the first lens and establish a support structure around it." Then, you need to print the second lens within that support structure. But in porous silicon, you can hang the two lenses together. In this sense, integration is more seamless.

By using this method, a larger area of image can be reconstructed from a mixed achromatic microlens array. This array can capture light field information, which is a significant challenge for traditional polymer microlenses as they are typically not achromatic, paving the way for applications such as light field cameras and displays.

Source: Laser Net